Structure of selenium incorporated in pyrite and mackinawite as determined by XAFS analyses

Selenium has a toxic potential leading to diseases by ingestion and a radiotoxic potential as (79)Se radionuclide if discharged from a high-level nuclear waste repository in deep geological formations into the biosphere. Selenium is often associated with sulfides, such as pyrite, the most important near-surface iron sulfide and constituent of host rocks and bentonite backfills considered for radioactive waste disposal. This study was aimed at investigating the incorporation of Se(2-) and Se(4+) into pyrite and mackinawite to determine the relevance of iron sulfides to Se retention and the type of structural bonding. The syntheses of pyrite and mackinawite occurred via direct precipitation in batches and also produced coatings on natural pyrite in mixed-flow reactor experiments (MFR) under anoxic conditions at Se concentrations in the solutions of up to 10(-3) mol/L. Mineralogical analyses by SEM and XRD reveal the formation of pyrite and mackinawite phases. The average Se(2-) uptake in pyrite in batch experiments amounts to 98.6%. In MFR syntheses, it reaches 99.5%, both suggesting a high potential for retention. XAFS results indicate a substitution of sulfur by selenide during instantaneous precipitation in highly supersaturated solutions only. In selenide-doted mackinawite S(2-) was substituted by Se(2-), resulting in a mackinawite-type compound. S(-) is substituted by Se(-) in selenide-doted pyrite, yielding a FeSSe compound as a slightly distorted pyrite structure. Under slighter supersaturated conditions, XAFS results indicate an incorporation of Se(2-) and Se(4+) predominantly as Se(0). This study shows that a substitution of S by Se in iron sulfides is probable only for highly supersaturated solutions under acidic and anoxic conditions. Under closer equilibrium conditions, Se(0) is expected to be the most stable species.